Full Idea
Now if we would be willing to subtract, in thought, the very least we can from these multitudes, must not that which is subtracted, too, be a multitude and not one, if it doesn't partake of the one?
Gist of Idea
If we subtract a part from a multitude, will that part not itself be a multitude?
Source
Plato (Parmenides [c.366 BCE], 158c)
A Reaction
This seems to be remarkably close to Dedekind's famous and widely accepted definition of infinity in Idea 9826.
Book Reference
Plato: 'Parmenides', ed/tr. Gill,M.L./Ryan,P. [Hackett 1996], p.166
Related Idea
Idea 9826 A system S is said to be infinite when it is similar to a proper part of itself [Dedekind]